add facility for browsing documentation

systemd-mode.el

@@ -22,10 +22,18 @@
 
 ;;; Code:
 
+(require 'thingatpt)
+(require 'url-parse)
+
 (defgroup systemd ()
   "Major mode for editing systemd units."
   :group 'tools)
 
+(defcustom systemd-browse-url-function 'eww
+  "Browser to use for HTTP(S) documentation."
+  :group 'systemd
+  :type '(choice function))
+
 (defcustom systemd-comment-start "#"
   "String to insert to start a new comment."
   :group 'systemd
@@ -53,6 +61,48 @@
   "Default expressions to highlight in `systemd-mode'. See systemd.unit(5)
 for details on unit file syntax.")
 
+(defun systemd-get-value (start)
+  "Joins lines in the key value starting at buffer position START,
+possibly broken by a backslash, and returns a string containing
+the value."
+  (save-excursion
+    (let ((break "\\\\\n")
+          end)
+      (while (progn (goto-char (- (line-end-position) 1))
+                    (looking-at break))
+        (forward-line))
+      (setq end (line-end-position))
+      (replace-regexp-in-string break " " (buffer-substring start end)))))
+
+(defun systemd-doc-find ()
+  "Find the value of the unit's “Documentation” keys and return
+as a list of strings, otherwise nil."
+  (let ((key "^Documentation=")
+        string)
+    (save-excursion
+      (goto-char (point-min))
+      (while (re-search-forward key nil t)
+        (setq string (concat string " " (systemd-get-value (point))))))
+    (when string
+      (split-string string))))
+
+(defun systemd-doc-open (url)
+  "Open URL.  Interactively completes the documentation in the
+current unit file, defaulting to the link under point, if any."
+  (interactive
+   (let* ((uri (thing-at-point-url-at-point))
+          (prompt (concat "URL"
+                          (when uri (format " (default %s)" uri))
+                          ": ")))
+     (list (completing-read prompt (systemd-doc-find) nil nil nil nil uri))))
+  (let ((link (url-generic-parse-url url)))
+    (pcase (url-type link)
+      ("file" (find-file (url-filename link)))
+      ("man" (url-man link))
+      ("info" (url-info link))
+      ((or "http" "https") (apply systemd-browse-url-function `(,url)))
+      (_ (user-error "Invalid link")))))
+
 (defvar systemd-mode-syntax-table
   (let ((table (make-syntax-table)))
     (modify-syntax-entry ?\" ".   " table)
@@ -61,6 +111,12 @@ for details on unit file syntax.")
     table)
   "Syntax table used in `systemd-mode' buffers.")
 
+(defvar systemd-mode-map
+  (let ((map (make-sparse-keymap)))
+    (define-key map (kbd "C-c C-o") 'systemd-doc-open)
+    map)
+  "Keymap used in `systemd-mode' buffers.")
+
 ;;;###autoload (add-to-list 'auto-mode-alist '("\\.automount\\'" . systemd-mode))
 ;;;###autoload (add-to-list 'auto-mode-alist '("\\.busname\\'" . systemd-mode))
 ;;;###autoload (add-to-list 'auto-mode-alist '("\\.mount\\'" . systemd-mode))
@@ -77,7 +133,10 @@ for details on unit file syntax.")
   "Major mode for editing systemd unit files. See
 http://www.freedesktop.org/wiki/Software/systemd/ for more
 information about systemd.  The hook `systemd-mode-hook' is run
-at mode initialization."
+at mode initialization.
+
+Key bindings:
+\\{systemd-mode-map}"
   (set-syntax-table systemd-mode-syntax-table)
   (setq-local comment-start systemd-comment-start)
   (setq-local font-lock-defaults '(systemd-font-lock-keywords))